Topical delivery of drugs is a very useful method for achieving systemic or localized pharmacological effects, although there is a main challenge involved in providing sufficient drug penetration across the skin. Skin consists of multiple layers, in which the stratum corneum layer is the outermost layer, then a viable epidermal layer, and finally a dermal tissue layer. The thin layer of stratum corneum represents a major barrier for chemical penetration through the skin. The stratum corneum is responsible for 50%-90% of the skin barrier property, depending upon the drug material's water solubility and molecular weight.
An alternative to the use of hypodermic needles for drug delivery by injection is disclosed in U.S. Pat. No. 3,964,482 (by Gerstel), in which an array of either solid or hollow microneedles is used to penetrate through the stratum corneum and into the epidermal layer. Fluid is dispensed either through the hollow microneedles or through permeable solid projections, or perhaps around non-permeable solid projections that are surrounded by a permeable material or an aperture. A membrane material is used to control the rate of drug release, and the drug transfer mechanism is absorption.
Other types of microneedle structures are disclosed in WO 98/00193 (by Altea Technologies, Inc.), and in WO 97/48440, WO 97/48441, and WO 97/48442 (by Alza Corp.). In addition, WO 96/37256 discloses another type of microblade structure.
The use of microneedles has one great advantage in that intracutaneous drug delivery or drug sampling can be accomplished without pain and without bleeding. As used herein, the term “microneedles” refers to a plurality of elongated structures that are sufficiently long to penetrate through the stratum corneum skin layer and into the epidermal layer. In general, the microneedles are not to be so long as to penetrate into the dermal layer, although there are circumstances where that would be desirable. Since microneedles are relatively difficult to manufacture, it would be an advantage to provide methodologies for constructing microneedles that are made from various types of micromolds that can be manufactured relatively quickly. The use of metallic molds or semiconductor molds is possible, but such structures usually take a relatively long period of time for construction. On the other hand, if the molds are made of a polymer or other type of plastic (or other moldable) material, then such mold structures can be made relatively quickly and with much less expense.